In recent years, a display surface for a projection apparatus or a display apparatus is usually implemented with a touch function. Instead of using the mechanical button for a specific operation, a user can operate with the displayed or projected images by directly touching the display surface with an object or his/her finger. Basically, the touch screens can be divided into resistive, capacitive, and optical types. The optical touch screen uses an optical sensing element to receive the beams reflected by the finger or the object and determines the position of the finger or the object on the touch screen or performs the corresponding operation instructions.
The optical touch system performs the touch operation by analyzing the beam reflected by the user's finger or the object and determining the position of the finger or the object on the touch screen. In order to provide suitable touch sensitivity, the light curtain has to be disposed with a proper distance with respect to the display surface in the optical touch system. If the distance between the light curtain and the display surface is relatively large, a part of the light curtain is reflected and detected by the optical touch system even when the user's finger or the object is relatively away from the screen. As a result, the optical touch system may have too high touch sensitivity. On the contrary, if the distance between the light curtain and the display surface is relatively small, the light curtain will not be reflected and detected by the optical touch system unless the user's finger or object is very close to the screen. As a result, the optical touch system may have too low touch sensitivity. It is quite inconvenient for a user to operate the optical touch system no matter the touch sensitivity of the optical touch system is too high or too low.
Moreover, when the screen of the optical touch system is disposed on a tilted wall, the light curtain generated by a light-emitting device may be also tilted with respect to the display surface of the screen. The touch sensitivity of the optical touch system may therefore be affected, which would be inconvenient to a user for operation.
Publications related to optical touch systems have been disclosed, such as US patent application No. 2011/0083109.
The information disclosed in this “BACKGROUND OF THE INVENTION” section is only for enhancement understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Furthermore, the information disclosed in this “BACKGROUND OF THE INVENTION” section does not mean that one or more problems to be solved by one or more embodiments of the invention were acknowledged by a person of ordinary skill in the art.